libiptc: add Libs.private to pkgconfig files

This is needed when doing static linking.
(pkg-config --static --libs libiptc)

References: http://bugzilla.netfilter.org/show_bug.cgi?id=675
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/libiptc.pc.in b/libiptc.pc.in
index 63a459a..99a3544 100644
--- a/libiptc.pc.in
+++ b/libiptc.pc.in
@@ -8,4 +8,5 @@
 Description:	iptables ruleset ADT and kernel interface
 Version:	@PACKAGE_VERSION@
 Libs:		-L${libdir} -liptc
+Libs.private:	-lip4tc -lip6tc
 Cflags:		-I${includedir}